Field of the Invention
The present invention relates to an image forming apparatus including a mechanism for applying a lubricant to an image-bearing member.
Description of the Related Art
Image forming apparatuses, such as copiers, printers, and fax machines, using photolithography include intermediate transfer tandem system image forming apparatuses each including an intermediate transfer belt to which a toner image on a photosensitive drum is transferred while the belt is rotating in contact with the photosensitive drum. Some of these image forming apparatuses each include a cleaning blade made of an elastic material, such as urethane rubber, as a cleaning unit for cleaning transfer residual toner or the like on an image-bearing member, e.g., the photosensitive drum or the intermediate transfer belt. Some of the image forming apparatuses including the cleaning blade are configured such that smooth sliding between the image-bearing member and the cleaning blade is maintained using a developer, such as toner or an external additive contained in the toner. In such a configuration, a charge amount of the developer tends to vary depending on ambient temperature and humidity. If the amount of developer reaching the cleaning blade markedly decreases, the smooth sliding of the cleaning blade will fail to be maintained. Unfortunately, this may cause a phenomenon (called “blade turn-up”) in which the blade reversely turns from its end as a start point.
Image forming apparatuses recently developed to stabilize smooth sliding are configured such that a solid lubricant, as represented by zinc stearate, is supplied (or applied) to the image-bearing member. In such a configuration for lubricant application, a fur brush (rotating brush) of nylon or the like against which the solid lubricant is pressed is in contact with the rotating image-bearing member, thus applying the lubricant to a surface of the image-bearing member with the fur brush which is slightly scraping away the lubricant.
The amount of solid lubricant needed varies because stability at the nip between the cleaning blade and the image-bearing member changes depending on operating conditions or the like of the image forming apparatus. For example, an image forming apparatus including a supply amount adjustment unit for changing the amount of lubricant has recently been developed (refer to Japanese Patent Laid-Open No. 2007-140085). In this image forming apparatus, a lubricant supply device for supplying a lubricant to a surface of any one of a photosensitive drum, an intermediate transfer belt, and a secondary transfer roller, serving as a target member that bears a toner image thereon, adjusts the amount of lubricant supplied by changing the number of rotations of an applying roller or pressure. When a new solid lubricant is attached, or alternatively, when a condition where the area of output images is large (or image density is high) is continued, control is performed such that the amount of lubricant is increased by increasing the number of rotations of the applying roller, for example.
In addition, an image forming apparatus known in the art is configured such that the amount of solid lubricant applied is changed depending on the amount of toner detected by a sensor unit (refer to Japanese Patent Laid-Open No. 2008-116547). In this image forming apparatus, control is performed such that the amount of lubricant applied is increased in response to detection of a large amount of toner by the sensor unit.
Another image forming apparatus known in the art is configured such that the amount of lubricant applied is changed in accordance with stored image forming information (refer to Japanese Patent Laid-Open No. 2009-15229). In this image forming apparatus, control is performed such that the number of rotations of an applying brush is increased at the start of image formation and the number of rotations is reduced over time during image formation. In addition, control is performed such that when image forming information indicates an early stage of image formation, the number of rotations is set to a small value and the number of rotations is increased over time.
Still another image forming apparatus known in the art includes an application amount control mechanism that controls a lubricant applying unit to change the amount of lubricant applied in accordance with the magnitude of rotational torque of a photosensitive drum (refer to Japanese Patent Laid-Open No. 2004-325924). In this image forming apparatus, control is performed such that the amount of lubricant applied is increased when the rotational torque of the photosensitive drum is greater than a reference value.
As regards an image-bearing member to contact a printing medium, such as a sheet of paper, when a lubricant is applied to the image-bearing member, only part of the lubricant applied to the image-bearing member is supplied to a cleaning blade during sheet supply because the lubricant is transferred to the printing medium. In some cases, control for extending a sheet-to-sheet interval (interval between printing media conveyed) during waiting for adjustment of fixing temperature (temperature adjustment) or waiting for conversion of image data is performed in order to maintain productivity. In such a case, substantially the whole of the lubricant supplied to the image-bearing member is supplied to the cleaning blade.
If the lubricant is excessively supplied to the cleaning blade, frictional force acting between the cleaning blade and the image-bearing member will increase too much. Disadvantageously, this may cause vibration (hereinafter, referred to as “blade chattering”) of the cleaning blade or blade turn-up. Furthermore, during downtime for sheet-to-sheet interval automatic adjustment (e.g., density adjustment) with no sheet supply, the lubricant on the image-bearing member is not absorbed by a printing medium. Disadvantageously, this may cause excessive supply of the lubricant.
Study results demonstrated that the amount of lubricant taken away from an image-bearing member varies depending on the type of paper supplied. Whereas paper having a smooth surface taken a large amount of lubricant away from the image-bearing member, paper having a rough surface, in particular, recycled paper having a smoothness less than 20 s, taken a small amount of lubricant away from the image-bearing member. The smoothness was measured in conformity with Japanese Industrial Standards (JIS) P 8119:1998.
Furthermore, the amount of lubricant taken away from the image-bearing member also varies depending on the material of paper supplied. An overhead transparency (OHT) sheet takes less lubricant away from the image-bearing member than plain paper. The OHT sheet and recycled paper may cause a large amount of lubricant to be supplied to the cleaning blade. In supplying thick paper or OHT sheets, control for reducing process speed (peripheral velocity) is typically performed in order to increase fixability. A constant ratio of linear velocity of an applying member to process speed results in a substantially constant amount of lubricant applied to the image-bearing member. Accordingly, if process speed of the image-bearing member is reduced, rotation speed of the applying member can also be reduced.
Under the above-described circumstances, Japanese Patent Laid-Open Nos. 2007-140085, 2008-116547, 2009-15229, and 2004-325924 describe that the amount of lubricant applied is adjusted depending on a certain condition, but do not take account of excessive supply of a lubricant to a cleaning blade caused during, for example, downtime, supply of different printing media, or an image-bearing member idle rotation mode.